adam's notes

Hubs

A hub is a legacy network hardware device that acts like a multiport repeater so that every port receives transmissions sent from any other port.

  • aka repeater
  • used to implement the 10BASE-T and 100BASE-T Ethernet cabling designs
    • design is referred to as a star topology because each end system is cabled to a concentrator (the hub)

How it Works

  • works at the physical layer
  • hub typically has between 4-48 ports
    • each computer is cabled to one port
  • The circuitry in the hub repeats an incoming transmission from a computer attached to one port across all the other ports
    • electrically, network segments looks like a single length of cable
  • Each computer attached to a hub receives all the traffic sent by other connected devices
    • referred to as a collision domain
    • every hub port is part of the same shared media access area and same collision domain
    • Each computer will ignore any frames that do not match its MAC address
    • However, when lots of computers are in the same collision domain, performance is reduced
      • as only one computer can send a frame at any one time
      • If two computers try to send at the same time, there is a collision, and they must wait for a random period before trying again
      • The more computers there are, the more collisions

Weaknesses

  • all node interfaces are half-duplex
    • means that the computer can send or receive, but not at the same time
    • use CSMA/CD protocol
  • media bandwidth is shared between all nodes
    • 10 Mbps or 100 Mbps
  • no hubs are compatible with Gigabit Ethernet
  • These limitations mean that almost all networks are now based on Ethernet switching
  • only likely to encounter a hub being used in very specific circumstances, such as where legacy equipment must be kept in service

Security Weakness of Hubs

  • a broadcast transmission is sent to all hosts in the same logical network area
    • accomplished by using the broadcast MAC address (ff:ff:ff:ff:ff:ff)
  • a unicast transmission is addressed to a single host only
    • uses host’s MAC address
  • with hubs, all interfaces receive all unicast and broadcast transmissions
    • hosts are typically configured to ignore unicast transmissions not addressed to them
    • setting an interface to promiscuous mode allows a host to capture (sniff) all unicast transmissions

MDI and MDIX

  • when Ethernet is wired to a hub, need to distinguish the interface on an end system from the intermediate system (hub)
    • end system interface is called medium dependent interface (MDI)
    • hub interface is called MDI crossover (MDIX)
    • means that transmit (Tx) wires on the host connect to the receive (Rx) wires on the hub
  • are no configuration options for a hub
    • just connect and power

Graph View

Backlinks